Among petroleum products, for example, lubricants, light gas oils, jet fuels, and the like are products that the cold flow property is considered to be important. Therefore, it is required that wax components, such as normal paraffin, which may degrade the low-temperature fluidity, and isoparaffin, which has a few branches, are entirely or partially eliminated from base oil used in such products, or that such wax components are converted into components other than wax components. In recent years, hydrocarbons prepared by the Fischer Tropsch synthetic process (hereinafter simply referred to as “FT synthetic oil”) has attracted attention as feedstock used in producing lubricants or fuels because FT synthetic oil contains no environmental burden-substance, such as sulfur compounds, however, such hydrocarbons contain a large content of wax components.
As a dewaxing technique for eliminating wax components from hydrocarbon oil, a method for extracting wax components by means of a solvent, such as liquefied propane or methyl ethyl ketone (MEK), has been known. However, in addition to requiring high costs, the following problems may arise in this method, such that the type of applicable feedstock is limited, and that the product yield is restricted by the type of the feedstock.
On the other hand, as a dewaxing technique for converting wax components contained in hydrocarbon oil into non-wax components, catalytic dewaxing has been known, for example, in which hydrocarbon oil is contacted with a catalyst known as a bifunctional catalyst, which has a hydrogenation-dehydrogenation function and an isomerization function, in the presence of hydrogen to isomerize normal paraffin contained in hydrocarbon into isoparaffin. As a bifunctional catalyst used in catalytic dewaxing, a catalyst containing molecular sieves including solid acids, particularly zeolites, and group 8 to 10 or group 6 metals of the periodic table has been known, and in particular, a catalyst in which the above-described metal is supported on the molecular sieve has been known.
Catalytic dewaxing is effective as a method for improving the low-temperature fluidity of hydrocarbon oil, and it is necessary to achieve a sufficiently high degree of conversion of normal paraffins in order to produce a fraction applicable as lubricant base oil and fuel base oil. However, the above-described catalyst used in catalytic dewaxing has a hydrocarbon cracking function in addition to an isomerization function, and therefore, in catalytic-dewaxing hydrocarbon oil, lightening of hydrocarbon oil may develop as the degree of conversion of normal paraffins rises, which makes it difficult to efficiently produce desired fractions. In particular, in producing high-quality lubricant base oil for which high viscosity index and low pour point are required, it is very difficult to economically produce a fraction to be prepared by catalytic-dewaxing hydrocarbon oil, and accordingly, synthetic base oils, such as poly-α-olefins, have often been used in the field concerned.
However, in recent years, in the field of producing lubricant base oil and fuel base oil, particularly in the field of producing lubricant base oil, production of group II, group III, and group III+ base oil, which are classified as grades of lubricants specified by the American Petroleum Institute (API), using hydroprocessing has become more and more widespread. Under these circumstances, in order to achieve a desired isoparaffin fraction from hydrocarbon oil including wax components with a high yield, a catalyst having an inhibited cracking activity and a high isomerization reaction activity on hydrocarbons, i.e., a hydroisomerization catalyst having an excellent isomerization selectivity, is required.
Attempts have been made to improve the isomerization selectivity of a catalyst used in catalytic dewaxing. For example, the following Patent Literature 1 discloses a process for producing a dewaxed lubricant, in which a straight-chain hydrocarbon raw material or a hydrocarbon raw material having a few branches having 10 or more carbon atoms is contacted, under an isomerization condition, with a catalyst which includes molecular sieves containing group 8 metals of the periodic table and having middle-size one-dimensional pores, the dimension of crystallite of which not exceeding 0.5μ, i.e., molecular sieves such as ZSM-22, ZSM-23, ZSM-48, and the like.
Note that a zeolite constituting a hydroisomerization catalyst is produced by hydrothermal synthesis in the presence of an organic compound known as “organic template”, which usually includes an amino group, ammonium group, and the like, to construct a predetermined pore structure. The synthesized zeolite is calcined at the temperature of 550° C. or higher, for example, in an atmosphere containing molecular oxygen to eliminate organic templates contained therein, as discussed in the final paragraph of “Item 2.1. Materials”, page 453 of the following Non Patent Literature 1. Typically, the calcined zeolite is then ion-exchanged to become an ammonium type zeolite in an aqueous solution containing ammonium ions, for example, as described in “Item 2.3. Catalytic experiments”, page 453 of the following Non Patent Literature 1. The ion-exchanged zeolite further carries group 8 to 10 metal components of the periodic table. The zeolite carrying the metal component is then charged into a reactor after being dried and having undergone steps such as extruding, and the like if necessary, and typically be calcined at a temperature of about 400° C. in an atmosphere containing molecular oxygen, and further undergoes reduction at a similar temperature by means of hydrogen and the like; thus a catalyst activity of a bifunctional catalyst is impaired.
In a recently proposed method, in order to further improve the isomerization selectivity of a hydroisomerization catalyst, a hydrothermally synthesized zeolite is ion-exchanged in a state in which organic templates are contained, instead of calcining such zeolite at the above-described high temperature, to produce a hydroisomerization catalyst based on the ion-exchanged zeolite (see the following Patent Literature 2, for example).